IMHO, All of this is circular thinking. Humans are social. Our species cannot exist without being social. Social groups cannot function without roles. Roles cannot exist without rules. Thus, social groups that did not form proper stratification simply failed and died out. Our ability to ask the question about the origin of the law stems from the fact that we are the functions of a process of social survival that stems back to the origins of our species.

In essence, our past perspectives on the law are irrelevant so long as they ensured survival. There is no "why" to them, any more than there is a "why" to our myriad of poorly designed biological evolutions. There are arguably equally many flavors of reason for law, but none of them have more core sense than the others...just differences in their aesthetic appeal. What matters is that they're compelling enough to keep enough people behind them; without that, we'd simply fail.